Grain boundaries with high Σ value and strain in grain matrix induce crack initiation in extruded 6000 series aluminium alloys

Abstract

The bending properties of 6000 series Al alloys are affected by the precipitates, shear band generation, preferred orientation, and residual stress distribution. Such alloys are characterized by initial cracks of extruded profiles that initiate at depths of 100–200 μm. Studies have used TEM to investigate the effect of precipitates. However, it is also important to evaluate the energetic stability of the grain boundaries and grain matrix around the crack origin at a micro level. To clarify the crack initiation mechanism, we considered the kernel average misorientation (KAM) and sigma (Σ) value at the crack initiation point. In cross sections from the surface region up to a depth of 400 μm, A6061 showed 15% more low-order Σ values of 3, 5, and 7, indicating that it had an energetically stable and strong interface compared to A6005C. The KAM map and Σ value of the surrounding area were obtained using electron backscatter diffraction measurements. The KAM map indicated that the initial crack occurred at the grain boundary with a high strain concentration and high Σ value. STEM-EDS results revealed precipitates in the grain matrix of AlMgSiCu-based Q or Q’ compounds. Their number density and PFZ width were almost the same, and they had only a small effect on crack initiation. However, the morphology of the precipitates in A6061 was smaller than that in A6005C; this may affect the strain distribution in the grain matrix. Thus, we focused on the energetic stability of the grain boundaries and grain matrix and found that cracks initiated at unstable grain boundaries with a large Σ value where strain was concentrated when a tensile force was applied.

Introduction

The 6000 series Al alloys are a type of age-hardened Al alloys to which Mg and Si are added to form Mg₂Si precipitates; A6061 and A6005C alloys are the specific subtypes that are often used in structural and vehicle members to meet the requirements of high yield strength, low deformation, and excellent corrosion resistance [[1], [2], [3], [4]]. Cu can be added to such alloys to improve their strength, and the addition of small amounts of transition elements like Mn and Cr can afford a grain refinement effect [5,6]. The shock absorption characteristics, that is, continuous deformation without undergoing fracture, have been evaluated through bending tests [7]. In alloys with high yield strength, a trade-off exists between tensile strength and bendability [8,9]; specifically, when the strength is increased, brittle fracture can occur, in turn degrading the shock-absorption performance. Moreover, the bendability is anisotropic because of the preferential orientation, i.e., the bendability is poorer in the transverse direction (TD) than in the extrusion direction (ED) [10].

The mechanical properties of these alloys are determined by the distribution and morphology of the precipitates and secondary phase particles at the grain boundaries and grain matrix. Bendability failure is generally caused by the following: shear band formation, texture distribution, precipitates, and/or secondary phase particles. Shear localization plays a major role in fracture initiation; precipitates and secondary phase particles act as a nucleation site for microvoids and degrade the bendability of Al alloys [[10], [11], [12]]. In addition, the alloy microstructure texture affects shear band formation [13].

Recrystallized textures usually depend on the shape of the extruded profile and process conditions such as the extrusion temperature, speed, and cooling rate. They are divided into two layers: surface and inner bulk. Rod-like and plate-shaped extruded profiles form a <100> recrystallized structure with cube and Goss orientation in the inner bulk, where the {001} face is aligned with the ED [14] and (112)[110], (001)[110], and (011)[100] appear at the surface layer [[15], [16], [17]]. The difference in recrystallization structures between the surface and the bulk is mainly induced by the shear stress caused by the friction at the die molding area and Al flow. Therefore, the texture formation mechanism in the extruded profile from the viewpoint of shear stress must be clarified to allow texture optimization and to thereby improve the mechanical properties by controlling the applied stress.

Our research group clarified the correlation between the residual stress and the texture distribution by performing XRD stress analysis using the cosα method [[18], [19], [20], [21]] with 3-axis oscillation, as shown in Fig. 1; doing so also clarified the macroscopic crack generation mechanism [22]. However, the strain distribution in crystal grains as expressed by the kernel average misorientation (KAM) map [23] and the energetic stability of the grain boundary as expressed by the sigma (Σ) value [24] at the nano-micro level have been insufficiently discussed. The KAM map has been used to visualize the strain state in the grain and thereby elucidate the plastic deformation and fracture mechanisms of Al and many other metals [25,26]. A low Σ value of the coincidence site lattice (CSL) boundary indicates a low grain boundary energy, representing high fracture and corrosion resistance, because grain boundary segregation depends on the grain boundary characteristics and structure [27,28].

The distribution state of precipitates affects the generation, and it must be investigated to clarify the initial crack generation mechanism, simultaneously including that in the precipitate-free zone (PFZ). The present study is the first to evaluate the origin of cracks in terms of the grain boundary stability and strain in grain matrix in consideration of precipitates. Furthermore, the crack initiation mechanism is schematically proposed from a micro-nano scale phenomena point of view around the grain boundary.